Field
Exemplary embodiments of the disclosure relate to a backlight unit using multi-cell light emitting diodes, and more particularly, to a backlight unit allowing low-current driving using multi-cell light emitting diodes configured to increase an effective luminous area of each light emitting cell.
Discussion of the Background
A liquid crystal display realizes an image by controlling transmittance of light emitted from a backlight unit. Although a cold cathode fluorescent lamp (CCFL) was generally used in the art, a light emitting diode (LED) has been generally used in recent years due to various advantages such as low power consumption, long lifespan, and environmental friendliness.
A light source for a liquid crystal display is classified into an edge type backlight unit and a direct type backlight unit depending upon the location of light emitting diodes. The edge type backlight unit is provided with a light guide plate and light emitting diodes acting as a light source are disposed on a side surface of the light guide plate such that light emitted from the light source is discharged to a liquid crystal panel through the light guide plate. Such an edge type backlight unit can reduce the number of light emitting diodes and does not require a low degree of quality deviation between the light emitting diodes, thereby enabling reduction in manufacturing costs and development of low power products. However, the edge type backlight unit has a limitation in the realization of high quality images due to difficulty in overcoming the difference in brightness between corners and a central region of the liquid crystal display.
On the other hand, the direct type backlight unit includes light emitting diodes disposed under a liquid crystal panel such that light can be directly supplied from a sheet light source having substantially the same area as the liquid crystal panel to an overall surface of the liquid crystal panel. As a result, the direct type backlight unit can minimize the difference in brightness between the corners and the central region of the liquid crystal panel while realizing high quality images.
However, in the direct type backlight unit, the light emitting diodes are densely arranged to achieve uniform back lighting with respect to a relatively large area, thereby causing damage to a drive circuit and deterioration in stability and reliability of the backlight unit resulting therefrom.
FIG. 1 is a block diagram of a typical backlight unit using LEDs and problems of the typical backlight unit will be described in more detail with reference to FIG. 1. Referring to FIG. 1, a typical backlight unit 1 includes a backlight control module 2 and a backlight module 5.
The backlight control module 2 includes a drive power generator 3 configured to generate and output DC drive power using input voltage Vin and a drive controller 4 configured to control operation of each of a plurality of LED arrays 6a to 6n which constitute the backlight module 5. Generally, the drive power generator 3 generates and outputs a DC voltage of 12 V, 24 V, 48 V, or the like.
The backlight module 5 includes the plurality of LED arrays 6a to 6n each including a plurality of LEDs connected to each other in series and an optical member (not shown) for improving luminous efficacy of light emitted from the plurality of LED arrays 6a to 6n. In FIG. 1, the backlight module 5 includes n LED arrays 6a to 6n connected to each other in parallel and each including 5 LEDs connected to each other in series. Here, the typical LEDs generally have a forward voltage level of 3V to 6.5V. Since it is difficult to control individual operation of such typical LEDs through connection to the drive power generator 3, the LEDs are connected to each other in series to constitute LED arrays such that each of the LED arrays can be individually driven and controlled. In such a typical backlight unit 1, the drive controller 4 may be configured to control brightness of all of the LED arrays 6a to 6n constituting the backlight module 5 through pulse width modulation (PWM) control of drive power supplied to the backlight module 5 in response to a dimming signal Dim input from the outside. Alternatively, in the backlight unit 1, the drive controller 4 may be configured to control brightness of a specific LED array among the LED arrays 6a to 6n by regulating the magnitude of drive current flowing through the specific LED array in response to a dimming signal Dim input from the outside.
Each of the LEDs used in such a typical backlight unit 1 is generally a single-cell LED and is driven by low voltage and high current. For example, the single-cell LED has a drive voltage of 3.6 V and can be operated by a drive current of 250 mA to 500 mA. Thus, in order to control operation of the backlight module 5 composed of such single-cell LEDs, peripheral circuits including the drive controller 4 are composed of large capacity electronic elements capable of handling high current, thereby causing increase in manufacturing costs. In addition, the peripheral circuits including the drive controller 4 can be damaged due to high-current driving of the aforementioned typical single-cell LED, thereby causing deterioration in stability and reliability of the backlight unit 1. Moreover, the high-current driving of the single-cell LEDs cause increase in power consumption and a droop phenomenon.